The present invention relates to new methylene-bridged metallocenes, to the corresponding ligands, to a new process for their preparation and to the use of said metallocenes as catalytic components in the polymerization of olefins.
Stereorigid chiral metallocene compounds possessing two bridged cyclopentadienyl groups condensed to a C5-C7 ring are well known in the state of the art and are mainly used as catalytic components in olefin polymerization processes; in particular, metallocene compounds possessing two bridged indenyl groups are widely used in the preparation of stereoregular polyolefins.
The numbering of the substituents on the indenyl group, to which reference is made in the present application, in accordance with the IUPAC nomenclature, is the following: 
In the bridged indenyl metallocene compounds known in the state of the art, the indenyl groups are linked together by divalent radicals containing one or more carbon atoms and/or heteroatoms; the divalent bridging groups are generally linked to the 1 position of said indenyl groups, and therefore, the common indenyl metallocenes are 1-indenyl compounds. For example, the European patent application EP 0 485 823 describes a class of bridged bis(1-indenyl) metallocenes, wherein the indenyl groups have a substituent other than hydrogen in the 2 position and are bridged in the 1 position by means of a bridge containing 1 or more carbon atoms (e.g. an ethylene or isopropylidene group) or containing heteroatoms (e.g. a dimethyl-silyl or a diphenyl-silyl group).
The European patent application EP 0 372 414 describes a very broad class of bridged or unbridged metallocenes; among the many metallocenes exemplified, two specific bis-indenyl metallocene compounds are reported, wherein the ethylene group bridging the indenyl groups is linked to the 1 position of one indenyl group and to the 2 position of the other indenyl group (formulae II-1 and II-2, on page 5 of said application).
The International patent application WO 94/11406 describes a very broad class of metallocene compounds of formula Rxe2x80x2 Ind-Mxe2x80x94(Cp)Qk, wherein: Ind is an indenyl group; Rxe2x80x2 is a substituent, other than hydrogen, linked in the 2 position of said indenyl group; Cp is a cyclopentadienyl group; M is a transition metal belonging to group 3, 4, 5 or 6 of the Periodic Table of Elements; and Q is a sigma-ligand of the metal M, k being an integer linked to the valence of M. Among the huge plethora of embodiments envisaged in the reported general formula, Rxe2x80x2 can form a bridge between the 2 position of the Ind group and the Cp group of the above formula; therefore, the class of bridged bis(2-indenyl) compounds is generically described. The definition of the bridging group Rxe2x80x2 is very broad too, the preferred bridges linking the two indenyl residues being hydrocarbon groups (preferably alkenyl or arylalkenyl groups) or groups containing at least one heteroatom belonging to Group 14, 15 or 16 of the Periodic Table of the Elements. In particular, working example XIV of this application describes the synthesis of methylene-bis(2-indenyl)zirconium dichloride, comprising numerous and laborious process steps. Said bis-indenyl zirconocenes have been tested in ethylene (co)polymerization (Examples XV-XIX) and give ethylene homopolymers in very low yield and ethylene/propylene copolymers having very low molecular weights and in low yields too. Furthermore, the same Applicant has demonstrated that methylene-bis(2-indenyl)zirconium dichloride is totally inactive in propylene polymerization, as will be described in the following. The European patent application EP 0 722 949 describes a process for preparing bis-cyclopentadienyl compounds bridged by a divalent CRIRII group, wherein RI is hydrogen or an alkyl radical, and RII is an alkyl or aryl radical. This process comprises the reactionlof a ketone or an aldehyde of formula RIRIICO, having the desired RI and RII groups, with a cyclopentadienyl compound, in the presence of a base and of an oxygen-containing solvent having an atomic ratio carbon/oxygen not higher than 3. However, the class of bis-cyclopentadienyl compounds obtainable with this process does not encompass compounds bridged with a methylene group; moreover, when RIRII CO is reacted with :an indenyl compound in the presence of a base, according to the above-mentioned process conditions, only bridged bis(1-indenyl) derivatives are obtained.
The European patent application EP 0 832 866 describes a process for preparing methylene-bridged bis-cyclopentadienyl compounds by reacting, in a two- or more-phases system, one or two cyclopentadienyl compounds with formaldehyde in the presence of a base and of a phase-transfer catalyst. Nevertheless, this application does not disclose any compounds wherein the two indenyl groups are linked in position 2; in fact, as evident from working example 1, this process leads to bis-(1-indenyl) derivatives and does not allow bridged bis(2-indenyl) compounds to be obtained.
WO 98/43931 (app. no. PCT/EP 98/01930), in the name of the same Applicant, describes a process for preparing methylene-bridged bis-cyclopentadienyl compounds, comprising the reaction of formaldehyde with a suitable cyclopentadienyl compound, in the presence of a base and a solvent having a dielectric constant, measured at 25xc2x0 C., higher than 7. Also in this case, the process allows only bis-indenyl compounds to be obtained, wherein the two indenyl moieties are bridged in position 1.
The Prins reaction (i.e., the acid-catalyzed condensation of carbonyl compounds and olefins) on indene has been described several times in the state of the art; as reported by Carleton W. Roberts (Friedel-Crafts and related reactions, vol., Part 2, page. 1192-1193, 1964, Interscience N.Y.), the reaction of indene with formaldehyde in the presence of acids does not give bis-indenyl products, but leads to the dioxane 1-hydroxy-2-hydroxymethylindan methylene ether as the major product. The same reaction was carried out by A. E. Gol""dovskii et al. (laslo-Zhir. Prom-st., 1987(8): 31; Chemical Abstracts 108:167399), who obtained said dioxane from indene and CH2O, in the presence of H2SO4.
From what reported above, it would be highly desirable to provide methylene-bridged bis(2-indenyl) derivatives, obtainable by means of an easy and advantageous route for the preparation thereof.
The Applicant has now unexpectedly found a new class of metallocenes, particularly active as catalyst components for the polymerization of olefins; said metallocenes are characterized by the presence of two indenyl groups bridged in the 2 position by means of a methylene group.
Therefore, an object of the present invention is a methylene-bridged metallocene of formula (I): 
wherein:
M is a transition metal belonging to group 3, 4, 5, 6 or to the lanthanide or actinide groups of the Periodic Table of the Elements (new IUPAC notation);
the substituents X, the same or different from each other, are monoanionic sigma ligands selected from the group consisting of hydrogen, halogen, xe2x80x94R, xe2x80x94OR, xe2x80x94OSO2CF3, xe2x80x94OCOR, xe2x80x94SR, xe2x80x94NR2 and xe2x80x94PR2 groups, wherein the R substituents are linear or branched, C1-C20, aliphatic hydrocarbon. C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl radicals optionally containing one or more atoms belonging to groups 13-17 of the Periodic Table of the Elements (new IUPAC notation), such as B, N, P, Al, Si, Ge, O, S and F atoms, and two R substituents may form a 5-7-membered ring; preferably, the substituents X are the same;
the substituents R1 and R2, the same or different from each other, are selected from the group consisting of linear or branched, C1-C20, aliphatic hydrocarbon, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing one or more atoms belonging to groups 13-17 of the Periodic Table of the Elements (new IUPAC notation; such as B, N, P, Al, Si, Ge, O, S and F atoms), xe2x80x94OR, xe2x80x94SR, xe2x80x94NR2, N-pyrrolyl, N-indolyl, xe2x80x94PR2, xe2x80x94BR2 and xe2x80x94SiR3 groups, wherein the R substituents have the meaning reported above, or two adjacent R2 substituents form a ring having from 4 to 8 carbon atoms;
p is an integer ranging from 0 to 3, being equal to the oxidation state of the metal M minus 2;
m is an integer ranging from 0 to 2; n is an integer ranging from 0 to 4;
with the proviso that, when m is 0, then n is different from 0.
Another object of the present invention is a catalyst for the polymerization of olefins comprising the product obtainable by contacting:
(A) one or more methylene-bridged metallocenes of formula (I), as described above; and
(B) a suitable activating cocatalyst.
Furthermore, the present invention provides a process for the polymerization of olefins comprising the polymerization reaction of one or more olefinic monomers in the presence of a catalyst as described above.
It is another object of the present invention a ligand of formula (II): 
wherein R1, R2, m and n have the meaning reported above.
The present invention further concerns a new and inventive process for the preparation of the above ligands of formula (II), comprising reacting formaldehyde or a derivative thereof with a indene of formula (III): 
wherein R1, R2 and n have the meaning reported above, and q is 0 or 1, in the presence of a Brxc3x8nsted acid, wherein the molar ratio between said compound (III) and formaldehyde is xe2x89xa71. When in the ligand of formula (II) m is xe2x89xa00, then the substituents R1 may also be introduced on the cyclopentadienyl ring by reacting a corresponding ligand of formula (II) having m=0 with a suitable amount of a deprotonating agent R3MgBr, R3MgCl or R3jB, wherein R3 may have the same meaning of R1; B is an alkaline or alkaline-earth metal, and j is 1 or 2, and then with a suitable amount of an alkylating agent R1Xxe2x80x2, wherein R1 have the meaning reported above and Xxe2x80x2 is halogen.
The methylene-bridged metallocenes of formula (I), the catalysts for the polymerization of olefins containing them, the ligands of formula (II) and the process for their preparation, according to the present invention, will be better described in the following detailed description.
It is an object of the present invention a methylene-bridged metallocene of formula (I), as reported above, wherein the metal M preferably belongs to group 4 of the Periodic Table of the Elements, and more preferably is Zr or Hf.
The X substituents are preferably Cl, Br or methyl, and are preferably the same.
R1 is preferably selected from the group consisting of methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, phenyl, benzyl, trimethyl-silyl and diphenylphosphino. The choice of the preferred R1 depends also on the nature of the final polymer, as will be evident from what reported below. R2 is preferably selected form the group consisting of halogen, methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, phenyl and benzyl.
The variable m is an integer ranging from 0 to 2; the variable n is an integer ranging from 0 to 4.
Non-limiting examples of methylene-bridged metallocenes corresponding to formula (I), according to the present invention, are:
rac- and meso-methylene-bis(1-methyl-2-indenyl)zirconium dichloride and dimethyl,
methylene-bis(1,3-dimethyl-2-indenyl)zirconium dichloride and dimethyl,
methylene-bis(4,7-dimethyl-2-indenyl)zirconium dichloride and dimethyl,
rac- and meso-methylene -bis(1-ethyl-2-indeny l)zirconium dichloride and dimethyl,
rac-methylene-bis(1-t-butyl-2-indenyl)zirconium dichloride and dimethyl,
rac- methylene-bis(1-t-butyl-2-indenyl)hafnium dichloride and dimethyl,
rac- and meso-methylene-bis(5-t-butyl-2-indenyl)zirconium dichloride and dimethyl,
rac- and meso-methylene-bis(1-trimethylsilyl-2-indenyl)zirconium dichloride and dimethyl,
rac- and meso-methylene-bis(5-trimethylsilyl-3-methyl-2-indenyl)zirconium dichloride and dimethyl,
rac- methylene-bis(1-phenyl-2-indenyl)zirconium dichloride and dimethyl,
rac- methylene-bis(-benzyl-2-indenyl)zirconium dichloride and dimethyl,
rac- and meso-methylene-bis(4-phenyl -2-indenyl)zirconium dichloride and dimethyl,
rac-methylene-bis(1-PPh2-2-indenyl) zirconium dichloride and dimethyl,
rac-methylene-bis(1-B(cyclohexyl)2-2-indenyl) zirconium dichloride and dimethyl, and
rac- and meso-methylene-bis(4-t-butyl-7-methyl-2-indenyl)zirconium dichloride and dimethyl.
The methylene-bridged metallocenes of formula (I) can be prepared by reaction of the corresponding ligands of formula (II) first with a compound capable of forming a delocalized anion on the cyclopentadienyl ring, and then with a compound of formula MXp+2, wherein M, X and p are defined as above, according to common procedures known in the state of the art. When, in the metallocene of formula (I), one or more X groups are other than halogen, it is necessary to substitute one or more halogens Z of the metallocene halide, obtained as reported above, with one or more substituents X other than halogen. The substitution reaction can be carried out by standard procedures, for example, when the substituents X are alkyl groups, by reacting the metallocene halide with alkylmagnesium halides (Grignard reagents) or with alkyllithium compounds.
According to another embodiment, when in formula (I) the X groups have the meaning of xe2x80x94R, as defined above, the methylene-bridged metallocenes of the invention can be obtained by reacting directly a ligand of formula (II) with at least one molar equivalent of a compound of formula MXs, in the presence of at least (p+2) molar equivalents of a suitable alkylating agent, wherein R, M and X have the meaning reported above and s is an integer corresponding to the oxidation state of the metal M and ranges from 3 to 6. Said alkylating agent can be an alkaline or alkaline-earth metal, such as LiR or MgR2, or a Grignard reagent, such as RMgCl or RMgBr, as described in WO 99/36427 (priority European app. no. 98200077.0), in the name of the same Applicant.
It is another object of the invention a ligand having the above-mentioned formula (II), wherein R1, R2, m and n have the meaning reported above. According to the present invention, the ligands of formula (II) are prepared by means of a new process, particularly simple and efficient, using cheap starting materials and comprising a single reaction step with very high yields. Furthermore, said process does not require laborious and time-consuming purification procedures, being particularly suitable to large-scale production.
The process of the invention comprises reacting formaldehyde or a derivative thereof with an indene of formula (III), as reported above, in the presence of a Bronsted acid, wherein the molar ratio between the compound (III) and formaldehyde is xe2x89xa71.
Said process is preferably carried out in an aprotic solvent, which may be polar or apolar; a preferred polar solvent is toluene, while a preferred apolar solvent is n-decane. Said Bronsted acid is preferably selected from the group consisting of p-toluensulphonic acid, acetic acid, methanesulphonic acid, HCl, H2SO4, HBF4 and mixtures thereof. Formaldehyde is preferably used in the form of a solution in water (formalin) or in the polymeric form (parafotrmaldehyde). However, other forms of formaldehyde as well as its solutions at different concentrations can be suitably used. Suitable derivatives of formaldehyde to be used in the process of the invention are compounds able to release formaldehyde under reaction conditions; preferred derivatives of formaldehyde are 1,3-dioxolane and trioxane. According to the process of the invention, formaldehyde or a derivative thereof can be reacted with an indene of formula (III) at any temperature above the melting point of the solvent. The temperature is preferably lower than the boiling point of the solvent, and more preferably is comprised between about 50xc2x0 C. and 100xc2x0 C.
The reaction time is not limiting, depending to a large extent on the acid, solvent and substrate used, on the concentration of the reactants and on the temperature of the reaction.
The molar ratio between said indene of formula (III) and formnaldehyde or a derivative thereof can vary over a wide range; said molar ratio is xe2x89xa71, and preferably is about 2.
The molar ratio between said acid and said indene of formula (III) can vary over a wide range; the molar ratio acid/indene (III) preferably ranges from 0.01 to 2, and more preferably from 0.1 to 1.5. The methylene-bridged ligands (II) obtained from the process of the present invention can be isolated according to standard procedures known in the state of the art, e.g. by recrystallization from a suitable solvent.
When in the ligand of formula (III) m is xe2x89xa00, the substituents R1 can also be introduced on the cyclopentadienyl ring by reacting the corresponding ligand of formula (II) wherein m=0 with a suitable amount of a deprotonating agent R3MgBr, R3MgCl or R3jB, wherein R3 may have the same meaning of R1, B is an alkaline or alkaline-earth metal, and j is 1 or 2, and then with a suitable amount of an alkylating agent R1Xxe2x80x2, wherein R1 and Xxe2x80x2 have the meaning reported above.
Preferably, the compound of formula (II) wherein m=0 is reacted with at least 2 equivalents of said deprotonating agent, at a temperature ranging from xe2x88x9278xc2x0 C. to 20xc2x0 C., for a time of 1-24 hours. The reaction is preferably conducted in an organic solvent selected from the group consisting of THF, Et2O, toluene and mixtures thereof. Said deprotonating agent is preferably selected from the group consisting of RLi and KH.
The process of the present invention has the advantage of allowing the synthesis of the new ligands (II) in a rapid and easy one-step reaction; these ligands could not be obtained or could be only hardly obtained, in very low yields and with laborious and time consuming procedures, according to the processes known in the state of the art.
The methylene-bridged metallocenes according to the present invention can be advantageously used as catalytic components for the polymerization of olefins. Thus, another object of the present invention is a catalyst system for the polymerization of olefins, comprising the product obtainable by contacting:
(A) one or more bridged metallocene compounds of formula (I), as described above, and
(B) a suitable activating cocatalyst.
Activating cocatalysts suitable as component (B) in the catalysts of the invention are linear, branched or cyclic alumoxanes, containing at least one group of the type: 
wherein the substituents R4, the same or different from each other, are a linear or branched, C1-C20, aliphatic hydrocarbon C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl, C7-C20 arylalkyl radicals, optionally containing Si and Ge atoms, or R4 is a group xe2x80x94Oxe2x80x94Al(R4)2. R4 is preferably methyl, ethyl, isobutyl or 2,4,4-trimethyl-pentyl.
Examples of alumoxanes suitable as activating cocatalysts in the catalysts according to the present invention are methylalumoxane (MAO), tetra-isobutyl-alumoxane (TIBAO), tetra-2,4,4-trimethylpentylalumoxane (TIOAO) and tetra-2-methyl-pentylalumoxane. Mixtures of different alumoxanes can also be used.
Activating cocatalysts suitable as component (B) in the catalysts of the invention are also the products of the reaction between water and an organometallic aluminum compound, preferably of formula AIR33 or Al2R56, wherein R5 has the meaning reported above. Particularly suitable are the organometallic aluminum compounds of formula (II) described in EP 0 575. 875 and those of formula (II) described in WO 96/02580. Moreover, suitable cocatalysts are the ones described in WO 99/21899 (priority European app. no. 97203332.8) and in the European app. no. 99203110.4. Non-limiting examples of organometallic aluminum compounds of formula AIR43 or Al2R46 are:
Particularly preferred aluminum compounds are trimethylaluminum (TMA), tris(2,4,4-trimethylpentyl) aluminum (TIOA), triisobutylaluminum (TIBA), tris(2,3,3-trimethyl-butyl)aluminum and tris(2,3-dimethyl-butyl)aluninum. Mixtures of different organometallic aluminum compounds and/or alumoxanes can also be used.
The molar ratio between aluminum and the metal M of the methylene-bridged metallocene is preferably comprised between about 10:1 and about 50,000:1, and preferably between about 100:1 and about 4,000:1.
In the catalyst system according to the present invention, both said methylene-bridged metallocene and said alumoxane can be pre-reacted with an organometallic aluminum compound of formula AIR43 or Al2R46, wherein the R4 substituents have the meaning reported above.
Further activating cocatalysts suitable as component (B) in the catalysts of the invention are those compounds capable of forming an alkylmetallocene cation; preferably, said compounds have formula Y+Zxe2x88x92, wherein Y+ is a Broensted acid capable of donating a proton and of reacting irreversibly with a substituent X of the compound of formula (I), and Zxe2x88x92 is a compatible non-coordinating anion, capable of stabilizing the active catalytic species which result from the reaction of the two compounds, and which is sufficiently labile to be displaceable by an olefinic substrate. Preferably, the Zxe2x88x92 anion comprises one or more boron atoms. More preferably, the anion Zxe2x88x92 is an anion of formula BAr4(xe2x88x92), wherein the Ar substituents, the same or different from each other, are aryl radicals such as phenyl, pentafluorophenyl, bis(trifluoro-methyl)phenyl. Tetrakis-pentafluorophenyl-borate is particularly preferred. Moreover, compounds of formula BAr3 can be conveniently used.
The catalysts of the present invention can also be used on inert supports. This is achieved by depositing the methylene-bridged metallocene (A), or the product of its reaction with the activating cocatalyst (B), or the component (B) and then the metallocene (A), on a suitable inert support, such as silica, alumina, magnesium halides, olefin polymers or prepolymers, such as polyethylenes, polypropylenes or styrene-divinylbenzene copolymers.
The thus obtained supported catalyst system, optionally in the presence of alkylaluminum compounds, either untreated or pre-reacted with water, can be usefully employed in gas-phase polymerization processes.
The present invention also provides a process for the homo or copolymerization of olefins, comprising the polymerization reaction of one or more olefinic monomers in the presence of a catalyst system as described above. Representative examples of olefinic monomers which may be used in the polymerization process of the invention are ethylene, alpha-olefins such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene, and non-conjugated diolefins such as 1,5-hexadiene.
The catalyst systems of the invention are particularly advantageous in ethylene and propylene homopolymerizations, where they exert higher activities when compared to prior art analogues. Moreover, by changing the substitution patterns of the methylene-bridged metallocenes of the invention, it is possible to obtain in high yields, at temperatures of industrial interest, polyethylenes and polypropylenes having intrinsic viscosity (I.V.) ranging from very low values (oils or waxes) to high values (I.V. greater than 1 dl/g). Therefore, an advantage of the metallocenes of the invention is that they allow polymers having a wide range of molecular weights to be obtained.
From propylene homopolymerization, by using the metallocenes of formula (I) wherein M is Zr, according to the invention, it is possible to obtain atactic propylene oligomers, terminated with vinylidene end-groups, which are particularly useful as lubricants, functionalizable monomers and chemical intermediates. By using the metallocenes of formula (I) wherein M is Hf, atactic polypropylene having higher molecular weight values are obtainable.
From ethylene homopolymerization, it is possible to obtain linear xcex1-olefins having a Pn (Number Average Degree of Polymerization) ranging from 50 to 500, and preferably from 80 to 350; these xcex1-olefins have more than 90% of terminal vinyl unsaturations (on the total number of terminal vinyl and vinylidene unsaturations). By varying the substitution pattern on the catalysts according to the invention, and in particular by using methylene-bridged bis(1-methyl-2-indenyl)metallocenes, it is possible to obtain linear xcex1-olefins having a percentage of terminal vinyl unsaturations higher than 90%, preferably higher than 95%, thus providing useful xcex1-olefinic PE waxes. Moreover said xcex1-olefins are linear, having a number of total branches preferably lower than 1/100 carbon atoms, and more preferably lower than 0.1/100C. These xcex1-olefins are particularly useful as polymerization monomers and chemical intermediates.
When the present catalyst systems are used in the copolymerization of ethylene and propylene, the methylene-bridged metallocenes of the invention allow to obtain copolymers in high yields, having a broad range of comonomer content.
The polymerization process can be carried out in the liquid phase, optionally in the presence of inert hydrocarbon solvents, or in the gas phase. The hydrocarbon solvent can be either aromatic, such as toluene, or aliphatic, such as propane, hexane, heptane, isobuitane and cyclohexane.
The polymerization temperature is generally comprised between xe2x88x92100xc2x0 C. and +150xc2x0 C., and more particularly between 50xc2x0 C. and 100xc2x0 C. The lower is the polymerization temperature, the higher are the molecular weights of the polymers obtained.
The molecular weight of the polymers can be further varied by changing the type or the concentration of the catalytic components or by using molecular weight regulators, for example hydrogen.
The molecular weight distribution can be varied by using mixtures of different metallocenes, or by carrying out the polymerization in several steps, that differ with respect to the temperatures of polymerization and/or the concentrations of molecular weight regulators.
An advantageous embodiment of the process for the polymerization of olefins, according to the present invention, is the use of a metallocene of formula (I) in combination with other metallocenes known in the state of the art, so to obtain polyethylenes with a well-defined bimodal distribution. More specifically, a metallocene of formula (I) able to produce PE waxes having low molecular weight, such as methylene-bridged bis(1-methyl-2-indenyl)metallocenes, may be used in mixture with one or more metallocenes known in the state of the art, able to yield polyethylenes having high molecular weights; by combining the above metallocenes, it is possible to obtain bimodal or multimodal polyethylenes which, despite the presence of the PE wax fraction of the invention (having very low molecular weight), do not have significant amounts of extractables.
The polymerization yields depend on the purity of the metallocene compound of the catalyst. The metallocene compounds obtained by the process of the invention can be used as they are, or they can undergo purification treatments.
The components of the catalyst can be brought into contact with each other prior to polymerization. The duration of contact is generally between 1 and 60 minutes, preferably between 5 and 20 minutes. The pre-contact concentrations for the metallocene component (A) are between 1 and 10xe2x88x928 mole/l, whereas for component (B) they are between 10 and 10xe2x88x928 mole/l. Precontact is generally effected in the presence of a hydrocarbon solvent and, if suitable, in the presence of small amounts of monomer.